There has been increased international focus on tax avoidance and tax havens since the financial crisis, with the Organisation for Economic Co-operation and Development recently focusing on closing down tax loopholes. These could include the so-called "Double Irish Dutch," where companies use Irish and Dutch subsidiaries to switch profits to lower tax jurisdictions.

"We thought that the British regime was going to tighten…and when the coalition government gave an undertaking that wouldn't be the case, we came back," Sorrell said.

WPP, one of the world's biggest advertising companies, announced slightly better than expected revenues of £2.57 billion ($4.57 billion) for the first quarter, a 1.5 percent rise – but currency headwinds knocked 8.1 percent off reported revenues. The advertising group, which is listed in London but has operations all over the world, felt the impact of sterling's strength in the last quarter of 2013 in its revenues.

Sorrell confirmed that "the biggest short-term issue" for the companies was the strength of sterling.

He also described the proposed merger of rivals Publicis and Omnicom as "a bit of a soap opera" and claimed WPP was benefiting from it.

He added that, out of the BRICS countries, India and Brazil were performing well but Russia and China were slowing. WPP is one of the companies often cited as a good indicator of overall economic performance.